There has been proposed an exhaust emission control device (see, for example, References 1 and 2) with selective reduction catalyst (NOx reduction catalyst) which causes nitrogen oxides (NOx) to selectively react with a reducing agent even in the presence of oxygen, the catalyst being incorporated in an exhaust passage of a diesel engine for a vehicle to thereby attain reduction in discharged NOx concentration; the reducing agent used is urea water capable of being more easily dealt with than ammonia.
In such exhaust emission control device, a nozzle (reducing-agent addition means) is arranged to add the urea water upstream of the catalyst. Addition of the urea water with a temperature of the catalyst being more than about 200° C. causes the urea water to be decomposed into ammonia and carbon monoxide, and NOx in the exhaust gas is reduced and purified by ammonia through the catalyst.
Any surplus ammonia is once adsorbed to the catalyst and then contributes to reduction treatment of NOx.                [Reference 1] JP 2002-161732A        [Reference 2] JP 2002-513110A        